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Janet L. Over the last years, research has established that a sexual selection exists and is widespread in the plant and animal kingdoms; b it does not necessarily entail sexual dimorphism; even hermaphrodites have it; c it does not require intelligence or a sophisticated sense of esthetics; even tapeworms and plants choose mates; and d it does not require brawn or even mobility for competition; plants may compete for pollinators, and broadcast spawning invertebrates may also compete for matings. Although discussions of sexual selection often focus on sexual dimorphism, several phenomena that are commonly associated with sexual selection are widespread and highly developed in hermaphrodites.
These phenomena include a bizarre and expensive courtship and copulatory behavior, b multiple mating and sperm competition, c rapid evolution of genitalia, d special structures associated with courtship, and e sexual polymorphism. The skewed breeding sex ratios associated with sequential hermaphroditism have long been recognized as contributory to sexual selection. In many simultaneous hermaphrodites, although the sex ratio at mating may be one to one, the actual reproductive sex ratio may also be skewed, creating a high potential for sexual selection. Reproductive biology in hermaphroditic taxa also involves a lot of complexity unknown in dioecious taxa, such as sex change, facultative sex allocation and conditional reciprocity that offers opportunities to enrich our understanding of sexual selection and to test the assumptions and predictions of theory.
Sexual selection has come to be seen as a keystone of Charles Darwin's theory of evolution by natural selection, being the exception that proves the rule that evolution proceeds through differential reproduction see Ghiselin b for discussion. Famously, Darwin developed the theory of sexual selection to for certain traits such as the weapons used in male-male competition for example, a stag's antlers or the ornaments used to attract members of the opposite sex for example, a peacock's tail which seemed to be very important in obtaining mates but unimportant otherwise.
In his view, where females and males share the same habitat, food sources, predators, and so on, sexually dimorphic characters must have evolved as a result of differential mating success. The first questions to address, then, are the nature of sexual selection and how it might apply to hermaphrodites. Sexual selection is a term that has meant different things to different people. In a recent review, Tim Clutton-Brock listed 9 different definitions of sexual selection and the list is not exhaustive see Table 1.
Of those definitions, 4 involve sexual dimorphism sex differences and 2 refer only to males. However, the essence of sexual selection as Darwin defined it is selection through competition for mates. This depends not on a struggle for existence, but on a struggle between the males for the possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. As Andersson has pointed out, this definition can be applied to all organisms, including plants, since as in ecology, the effect of competition will be the same whether it occurs as interference competition male-male combat, for example , scramble competition sperm competition; pollen competition , or indirectly such as competition to be chosen by females or by pollinators see Levitan ; Skogsmyr and Lankinen ; Delph and Ashman ; Thomson Andersson's general definition of sexual selection applies to all forms of hermaphrodites as well as taxa, such as protists, that lack anisogamy, making it appropriate to test hypotheses about the origins of sexual selection contrary to Grant Therefore, consideration of hermaphrodites shows us the weaknesses of definitions of sexual selection that are specific to sexually dimorphic traits or to males alone.
The next questions concern how to identify and measure sexual selection and what its sources may be in hermaphrodites. The ultimate explanation for the rule of male-male competition and female choice was seen by Darwin as anisogamy. In surprisingly modern arguments he traced the source of male eagerness and female coyness to 1 the motility of sperm vs. Therefore, as is so often the case, in his description and definition of sexual selection, Darwin identified most of the issues that occupy us today. Bateman hypothesized that males compete for mates and females do not because reproductive success in females is limited by the resources available for egg production, that is, the female gain curve plateaus, whereas the reproductive success of males is limited only by access to females, and the male gain curve is proportional to the of mates or eggs available.
This hypothesis has been very influential, but problems with it have begun to be identified see Hubbell and Johnson ; Arnold and Duvall ; Gowaty ; Leonard ; see Tang-Martinez and Ryder for discussion. Another factor that has been used to explain the pattern of male-male competition and female choice in dieocious animals Alexander and Borgia and the preferred sexual role in simultaneous hermaphrodites Leonard and Lukowiak is the control of fertilization.
Alexander and Borgia argued that a fundamental difference between males and females is that in general, females retain a greater degree of control over the fate of their gametes than do males. The arguments of Bill Eberhard and Patty Gowaty suggest that female control of fertilization may represent a form of mate choice. At present, a popular hypothesis is that multiple factors may be at work to set the stage for sexual selection see Shuster and Wade Table 2 lists factors that have been considered to be important in determining the strength and direction of sexual selection by producing differences between the sexes or sexual roles in variance in reproductive success and consequently skewed breeding sex ratio BSR.
The great difficulty is to determine which differences between the sexes are causes of sexual selection and which are effects. In hermaphrodites each individual acts as both male and female and can act as its own control. Identification of the preferred sexual role in hermaphrodites is one way of testing alternative hypotheses as to the source of sexual conflict see following sections; review in Leonard Variance in reproductive success has long been of interest as a measure of sexual selection Bateman ; Payne ; Fincke ; Raffetto and others ; etc. The theoretical upper limit to the intensity of selection on a population is given by Crow's Index; the ratio of the variance in progeny to the square of the mean of progeny Crow ; Shuster and Wade The use of Crow's Index as a basis for measuring the intensity of sexual selection Wade ; Shuster and Wade is an important development in sexual selection theory but the current usage, I mates Shuster and Wade , is inappropriate for hermaphroditic taxa because it focuses on male mating success as a fraction of the total.
With separate sexes, in each generation sexual selection acting on males will be separate from that acting on females. In either sequential or simultaneous hermaphrodites however, sexual selection on an individual comes from the total of its success in both sexual roles. An appropriate measure of sexual selection for hermaphrodites would therefore have to include variance in total reproductive success. Clearly, the intensity of sexual selection cannot be larger than total selection so that Crow's Index represents a theoretical upper limit to the strength of sexual selection.
In reality, sexual selection will seldom or never reach this upper limit since other forces of selection are expected to be in operation, although it might be possible to construct experimental conditions that would bring a population close. Also, it should be emphasized that the magnitude of the variance is a measure of the potential for selection and not a measure of the intensity of selection see discussion Wade ; Grafen ; random factors could also produce variance in reproductive success see Sutherland , However, differences in variance in reproductive success among populations with similar life histories and biology, would offer a first approximation to the potential for sexual selection.
Defining sexual selection as the product of competition with conspecifics for reproductive opportunities is clear in principle, however identifying and measuring it in practice is far from simple, as Darwin predicted. Despite the increasingly sophisticated approaches of quantitative genetics that are beginning to be employed Holland and Rice ; Shuster and Wade ; Mead and Arnold , most of sexual selection research still relies on the same sorts of evidence that attracted Darwin's attention; the morphology, sexual behavior, and social structure of animals.
Although the biology of many hermaphroditic taxa, particularly invertebrate animals, remains poorly known, review of the literature on hermaphrodite mating systems shows evidence for a variety of phenomena that are associated with sexual selection in dieocious organisms Table 3. Male-male competition has been the major focus of sexual selection research in dioecious species see Table 1 and among hermaphrodites has been long studied in plants in the form of pollen competition see reviews in Willson and Burley ; Delph and Havens ; Skogsmyr and Lankinen ; Delph and Ashman ; Thomson More indirect competition in the form of competition for pollinators has been an important evolutionary force in angiosperms and a very active field of research see reviews in Skogsmyr and Lankinen ; Thomson Less work has been done in animals, but there is strong circumstantial and some direct evidence to suggest that sperm competition is common and important in many internally fertilizing simultaneously hermaphroditic taxa, especially gastropods and flatworms see reviews in Baur ; Michiels The sperm-cast mating system of sessile hermaphroditic animals that brood eggs may lead to ificant levels of sperm competition see review in Bishop and Pemberton Multiple mating and sperm storage are common in hermaphroditic animals, including many androdieocious taxa see review in Weeks and others Direct evidence for sperm competition is available from a flatworm Pongratz and Michiels , a leech Tan and others , and the gastropods, H.
Some serranines also have harem polygamy see following section and Petersen and an increase in male mating opportunities seems to be associated with social dominance in this group Leonard Fighting between males has been reported in a sequentially hermaphroditic polychaete see discussion in Berglund , ; Sella and Ramella It seems likely that classic interference competition for access to mates occurs in other hermaphrodites, although documented examples are rare.
This may reflect either a lack of attention to the phenomenon or a difference in the biology of hermaphrodites. For example, in many androdioecious animals, males are rare relative to self-fertilizing hermaphrodites see review in Weeks and others so that competition among males for access to hermaphrodites may seldom occur. The prevalence of conditional reciprocity in mating systems in simultaneous hermaphrodites see discussion that follows and in Leonard may inhibit the success of interference competition. If this is true, species with hypodermic insemination, unilateral copulation without reciprocity, or chain copulation may be fruitful sources of evidence for direct interference competition.
Competition among hermaphrodites for access to a partner's sperm or for matings in the female role has not been studied but is theoretically possible see discussion in Leonard Female choice of mates is a key aspect of classical sexual selection. In an elegant series of studies, Webster and colleagues see review in Webster and Gower have demonstrated that Biomphalaria glabrata snails with resistant genotypes discriminate against mates on the basis of parasitic infection. There is also evidence that simultaneous hermaphrodites may discriminate among sperm on the basis of whether spermatophores were exchanged reciprocally, in an opisthobranch Karlsson and Haase and in the stylommatophoran Cantareus Helix aspersa , on the basis of whether a dart was received review in Koene although, in the latter case, it is not clear whether cryptic female choice or sperm competition is involved.
In a planarian, individuals gave more sperm to ly isolated individuals, suggesting a preference for lower sperm competition Michiels and Bakovski In some simultaneously hermaphroditic serranine fish, individuals with eggs to spawn mate preferentially with larger harem-holding hermaphrodites or pure males see reviews in Leonard ; Petersen Data on the of offspring sired by first vs.
Selective abortion in plants may represent a form of cryptic female choice see reviews in Willson and Burley ; Skogsmyr and Lankinen , although in plants female choice may be difficult to distinguish from parent-offspring conflict Skogsmyr and Lankinen Morphological sexual dimorphism is common in sequential hermaphrodites such as the well-studied examples of fish with social control of sex change for example, Warner and others ; Shapiro Monoecious plants, that is, those with separate male and female flowers on the same individual, may have sexually dimorphic flowers Barrett Ectoprocts with morphologically distinct male and female zooids in a hermaphroditic colony also may be considered to have sexual dimorphism see Bishop and Pemberton There is one report McLauchlan of stylommatophoran land snails with a shell dimorphism that allegedly reflects differences in sexual behavior associated with age and size.
What is more common in simultaneously hermaphroditic taxa than morphological sexual dimorphism is behavioral sexual dimorphism. That is, it is sometimes the case that individuals will behave quite differently when mating in one role rather than the other.
For example, the opisthobranch sea slug Navanax inermis copulates unilaterally with one individual acting as male and the other female and this is associated with distinct male and female courtship behaviors Leonard and Lukowiak , This is also the case in another opisthobranch, Aplysia californica Leonard and Lukowiak and in the basommatophorans Lymnaea stagnalis review in Koene , Physa acuta Ohbayashi-Hodoki and others and Biomphalaria glabrata Webster and Gower Simultaneously hermaphroditic serranine fish also show sexually dimorphic courtship behavior Fischer ; Petersen and in both Navanax and the serranines there is a preference for mating in one of the sexual roles with Navanax preferring the female role Leonard and Lukowiak ; Michiels and others and the serranines preferring the male role Leonard ; Petersen Such behavioral dimorphism will probably prove to be more widespread when more hermaphroditic taxa have been studied in detail.
In contrast, a species of stylommatophoran banana slug with unilateral intromission Ariolimax californicus has symmetrical courtship behavior Leonard and others The factors that for asymmetrical vs. Perhaps the most basic criterion for suspecting sexual selection in a species is the observation of sexual behavior or other reproductive characters that seem bizarre, or likely to be costly in terms of either energy, time or risk of harm to the possessor. Perhaps even more spectacular is the aerial mating of the land slug Limax maximus in which a pair of simultaneous hermaphrodites climb a tree or wall to an overhang, drop down on 10—25 cm of mucus strands and, hanging, exchange spermatophores between entertwined penes, without intromission Gerhardt ; Langlois The energetic cost of locomotion and mucus production in stylommatophorans is high Denny and mucus production is a major source of water loss in these terrestrial mollusks Luchtel and Deyrup-Olsen The mucus thre are often eaten after copulation, suggesting that the material content of the thre represents an important resource.
Limax is one of the many hermaphrodites that will repay investigation from the standpoint of sexual selection. Another bizarre behavior is the apophallation found in two species of Ariolimax slugs, whereby copulation is occasionally terminated by amputation of the penis of one or both individuals; Heath ; Mead ; Leonard and others A similar behavior has been reported for the genus Deroceras Rymzhanov with the difference that in this case, individuals are reported to amputate their own penis after a unilateral copulation and present it to the partner.
Michiels and Koene argue, on theoretical grounds, that sexual behavior involving damage to the individual acting as a female is particularly likely to evolve in hermaphrodites. One of the most important differences between sexual selection in dioecious vs.
That is, in each generation males compete with males for their contribution to the next generation, and females compete with females. Sexual conflict is an epiphenomenon. In hermaphrodites, in contrast, each individual competes with all others in the population, including its mates.
Since every individual has both a mother and father, in outbreeding populations both total and mean fitness must be equal for male and female functions. However, if the variances in fitness differ for the two sexual roles, then the potential fitness of the two sexual roles also differs, and one would expect that selection would favor individuals that specialize in the preferred role see Charnov ; Fischer , ; Leonard , , ; Michiels Based on Bateman's principle, Charnov predicted that simultaneous hermaphrodites should be more eager to mate as males than as females because of the potential for increased fitness; that is, simultaneous hermaphrodites should prefer to mate in the high variance sexual role, although data from angiosperms demonstrates that the female role in hermaphrodites may have higher variance than the male sexual role see Delph and Ashman for discussion.
Charnov's hypothesis is contrary to predictions from probability theory which demonstrate that where two strategies offer equal mean return, as must be the case for male and female functions in an outcrossing organism, the strategy with the lower variance will offer higher fitness due to the reduced probability of zero fitness Gillespie ; Real ; Leonard ; for recent review see Leonard This logic Gillespie's principle predicts that hermaphrodites will prefer the sexual role with lower variance see Leonard , for discussion.
If one sexual role has a lower variance than the other, that role should, by Gillespie's principle Gillespie ; contrary to Charnov ; see discussion in Leonard , , offer potentially higher fitness. Hermaphrodites able to specialize in that role, then, should have higher fitness and individuals should compete for that sexual role, creating a direct conflict of interest between mates see discussion in Leonard , This conflict of interest creates potential for sexual selection because a individuals compete for matings in one sexual role and b according to the Hermaphrodite's Dilemma model Leonard , individuals will choose mates that are willing to reciprocate by mating in both sexual roles see discussion in Leonard If there are systematic and consistent differences between the sexes in variance in reproductive success, then mating between simultaneous hermaphrodites should involve a conflict of interest whereby each individual will attempt to monopolize one sexual role and, since it takes two to tango or mate , mating systems based on conditional reciprocity represent a cooperative and stable solution to the problem Fischer ; Axelrod and Hamilton ; Leonard and Lukowiak ; Leonard ; see Leonard for review; discussion in Petersen The Hermaphrodite's Dilemma model Leonard predicts that where a consistent preference for one sexual role exists, mating systems will evolve to reciprocity Leonard ; see discussion in Leonard Many mating systems in pair-mating hermaphrodites involve either simultaneously or serially reciprocal mating.
In the few cases that have been studied, these mating systems appear to be based on conditional reciprocity for review see Leonard Direct evidence for conditional reciprocity is of two general types; the first is evidence that an individual's willingness to assume a particular sexual role is contingent on the partner's willingness to assume the same sexual role, resulting in either simultaneously or successively reciprocal mating see review in Leonard ; the second is evidence that an individual's readiness to mate in both sexual roles with a particular partner is dependent on the quality of that partner review in Leonard ; experimental data in Milinski ; Webster and Gower The mating systems of Ophyrotrocha spp.
Webster and Gower describe experiments demonstrating that in B. Petersen reviews the mating systems of serranines, which are based on conditional reciprocity, from the standpoint of game theory models see also Axelrod and Hamilton ; Leonard ; discussion in Leonard The available evidence from simultaneously hermaphroditic animals demonstrates a clear preference for the male sexual role in some taxa whereas the female sexual role is preferred in others see review in Leonard So far, data to link these role preferences with differential variance in reproductive success are not available but see Fischer A disadvantage of variance in reproductive success as a measure of potential for sexual selection is the difficulty of obtaining lifetime reproductive success for a large of individuals.
In hermaphrodites an additional problem is that of separating fitness due to male vs. Conditional reciprocity is not the only possible form of reciprocity. In non-pair mating simultaneous hermaphrodites, such as plants, and sessile invertebrates, for example, barnacles, ascidians and bryozoans Levitan ; Bishop and Pemberton ; Weeks and others , one might expect de facto reciprocity among individuals in a local population, which would satisfy the prediction of Hermaphrodite's Dilemma. The behavior of pollinators should be an important selective force in angioperms ThomsonLooking for hermaphrodite play
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